General Mechanical

General Mechanical

Acoustic Transmission Loss

    • peteroznewman
      Subscriber

      I made an Impedance Tube with a steel plate in the center of the air duct using ANSYS 19.1 Student.


      I coupled two systems together to get the FSI coupling between the air and the steel.



      I got the speaker side (Up) frequency response on the speaker side of the plate as shown here:



      While the Down side of the steel plate has this Frequency Response:



      These two curves let me calculate the Noise Reduction from the steel plate, which is the difference in SPL on each side, but I want to calculate the Transmission Loss.


      When I try to define a Port on the steel faces, I can't.  If I define Ports at the speaker and anechoic ends of the air duct, The Transmission Loss result won't accept them.  How do I calculate the Transmission Loss, which is defined below?



       


      Attached is an ANSYS 19.1 archive

    • sk_cheah
      Subscriber

      Hi Peter,


      I changed a few things in a new analysis branch. 


       


      Kind regards,
      Jason



       

    • peteroznewman
      Subscriber

      Hi Jason,


      Thanks for the information.  I was told that this model could be done in a single Harmonic Acoustics system, so I figured out how to add Physics Regions to make that happen. Once a Structural Physics Region is in the model, the FSI is automatically taken care of.  I created a stand alone system to reproduce your result. The tabular data is correct, but the Frequency axis on the Worksheet is wrong.



      Your model has the ports are at each end of the tube, a long way from where the incident and transmitted power is supposed to be measured at the plate . Since this is a lossless model, I suppose the power doesn't change along the tube, but I don't know why I can't use the face of the plate or the face of the air at the plate as a port.  Can you explain that?


      Kind regards,


      Peter


      ANSYS 19.1 archive is attached.

    • peteroznewman
      Subscriber

      I looked at the Transmission Loss of a steel plate with fixed edges.



      Below is the result for Transmission Loss of a Simply Supported edge on the steel plate



      ANSYS 19.1 Archive is attached.

    • sk_cheah
      Subscriber

      but I don't know why I can't use the face of the plate or the face of the air at the plate as a port.  Can you explain that?



      I'm not sure why but I'm guessing it's to do with how it calculates power. Experimentally, two microphones are needed to distinguish between incident and reflected waves.  


      As you've noticed, the transmission loss values is determined by the boundary condition and size of the plate (resonance of the plate). I believe that is why this method is typically used for specific mufflers and fibrous materials as opposed to general sound barriers where transmission loss changes with size and boundary conditions.


      Kind regards,
      Jason

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